Van Arkel de Boer method

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Apparatus made of quartz glass for the Van-Arkel-de-Boer process 1: to the vacuum pump, 2: 6 mm molybdenum electrode, 3: molybdenum mesh, 4: chamber for the raw metal, 5: 0.06 mm tungsten wire

The crystal bar process [ -buːr- ], also called growth process, is one of Anton Eduard van Arkel and Jan Hendrik de Boer in 1924 developed transport reaction for obtaining or for cleaning metals such. B. titanium , zirconium , hafnium , vanadium , tantalum , thorium , chromium , rhenium , protactinium or their compounds (e.g. titanium carbide TiC, titanium nitride TiN, zirconium nitride ZrN) and the semimetals boron , silicon .

process

Process scheme of the Van Arkel de Boer process (M = metal).

The metal to be cleaned is heated together with iodine at the bottom of an evacuated, bell-shaped vessel at a pressure reduced to 0.1-20 Pa. The reaction temperature for titanium is around 800 ° C.

The metal iodide, which is gaseous at this temperature, is formed in the forward reaction , in the case of titanium about the gaseous TiI 4 . This reaches a glowing tungsten wire through diffusion or convection , where it decomposes again. The pure metal separates out in the reverse reaction. The released iodine absorbs metal again. Any impurities previously present in the metal remain in the residue.

The process takes advantage of the different equilibrium between the forward and reverse reaction, which is exothermic with respect to the formation of iodide; i.e. at high temperature the equilibrium shifts to the left. This is entropically favored, which is why the reaction takes place under reduced pressure:

(Forward reaction: 600 ° C; reverse reaction: 1200 ° C)

The reaction is exothermic with Δ H  = −427 kJ / mol.

The method is also used when operating a halogen lamp . Due to the high operating temperature, sublimed tungsten atoms, which are deposited on the glass wall of the light bulb, are converted into WCl 4 or WI 4 by the small amounts of iodine and chlorine in the protective gas . At the significantly higher temperature on the filament itself, the equilibrium is shifted to the side of the educts, so that the elementary tungsten is deposited on the filament and thus reduces the loss of sublimation.

Products

The photos show high-purity crystal bars (metal wax bars ) made using the Van Arkel de Boer process.

  • Titan Crystal Bar, 99.995%

  • Zirconium Crystal Bar, 99.97%

  • Hafnium Crystal Bar,> 99.9%

  • Vanadium Crystal Bar, 99.9%

See also

literature

  • Erwin Riedel: Inorganic Chemistry. Walter de Gruyter, Berlin 2004, 6th edition

Individual evidence

  1. ↑ Entry on growing-up procedure. In: Römpp Online . Georg Thieme Verlag, accessed April 10, 2011.
  2. ↑ Yoshio Waseda, Minoru Isshiki (Ed.): Purification Process and Characterization of Ultra High Purity Metals . Springer, Berlin / Heidelberg 2002, ISBN 978-3-642-56255-6 , doi : 10.1007 / 978-3-642-56255-6 .
  3. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 91st – 100th, improved and greatly expanded edition. Walter de Gruyter, Berlin 1985, ISBN 3-11-007511-3 .
  4. ^ Script from TU Darmstadt ( Memento from February 26, 2007 in the Internet Archive ) (PDF; 602 kB).